1. Field of the Invention
This invention relates generally to narrow-band digital frequency-modulated (FM) limiter-discriminator (LD) receivers and, more particularly, to a digital FM LD receiver that averages the outputs of two detectors to eliminate the effects of FM-clicks on bit error rates (BERs) for encoded digital signals.
2. Description of the Related Art
The FM-click-and-Gaussian noise model for frequency-modulation (FM) receivers is well-known in the electronic arts, having been introduced by Rice in a seminal 1963 paper (S. O. Rice, xe2x80x9cNoise in FM receivers,xe2x80x9d Time Series Analysis, M. Rosenblatt, Ed., Wiley, N.Y., 1963, pp. 395-422) and discussed in scores of works over the years. The Rice noise model assumes that the FM receiver accepts a signal in additive noise that consists essentially of a continuous Gaussian component occasionally interrupted by a click.
The performance of a narrow-band limiter-discriminator (LD) digital FM system was first described by Tjhung and Wittke (T. T. Tjhung et al., xe2x80x9cCarrier transmission of binary data in a restricted band,xe2x80x9d IEEE Trans. Commun. Technol., vol. COM-18, pp. 295-304, August 1977) using the Rice noise model. Their approach was later simplified by Cartier (D. E. Cartier, xe2x80x9cLimiter-discriminator detection performance of Manchester and NRZ coded FSK,xe2x80x9d IEEE Trans. Aerosp. Electron. Syst., vol. AES-13, pp. 92-70, January 1977) and subsequently a complete analytical solution for the bit error rate (BER) in a narrow-band digital FM receiver was described by Pawula (R. F. Pawula, xe2x80x9cOn the theory of error rates for narrow-band digital FM,xe2x80x9d IEEE Trans. Commun., vol. COM-29, pp. 1634-1643, November 1981) for certain regions of time-bandwidth product and frequency-deviation ratio. All of these early studies assume an integrate-and-dump (IandD) bit detector and rely on the Rice FM-click noise model.
The FM-click and the continuous Gaussian elements of the system noise were found to contribute equally to the BER for a frequency deviation ratio of h=0.7, and the clicks were found to dominate BER when h greater than 0.7 and were found to be insignificant when h less than 0.7. Thus, the clicks were found to introduce a significant performance penalty in the optimum uncoded FM receiver using the integrate and dump (IandD) detector, for which the frequency deviation ratio, h=0.7 and the time-bandwidth product, BT=1.0.
In a later paper (R. F. Pawula, xe2x80x9cRefinements to the theory of error rates for narrow-band digital FM,xe2x80x9d IEEE Trans. Commun., vol. 36, pp. 509-513, April 1988), Pawula describes the theoretical performance of an uncoded narrow-band digital FM receiver using an IandD detector that operates over a fraction of the bit-interval. As the bit-interval fraction is reduced to zero, the IandD detector becomes in the limit a sample and hold (SandH) detector. Pawula found that a SandH detector exacted a performance penalty of 0.5 dB when substituted for an IandD detector. Later experimental efforts have shown excellent agreement with Pawula""s theoretical results for both the IandD and the SandH detector. Except as a limiting instance of a fractional-interval IandD detector, the SandH detector has been of little interest and has had little application in the art because of the 0.5 dB performance penalty.
A discussion of the Rice noise model by Bar-David and Shamai (I. Bar-David et al., xe2x80x9cOn the Rice model of noise in FM receivers,xe2x80x9d IEEE Trans. Inform. Theory, vol. IT-34, pp. 1406-1419, November 1988) observes that practitioners in the art had long known that FM-clicks are the major limitation to improved BERs in LD threshold detectors, even at high signal-to-noise ratios (SNRs). Because the click is the culprit, the stochastic properties of FM-clicks had been widely investigated. The individual clicks had been shown to have a Poisson distribution at high SNRs, as would be expected. Individual clicks had been shown to be statistically independent of the Gaussian component at high SNRs.
As exemplified by the Bar-David and Shamai paper, the literature is replete with reported efforts to extend the BER performance of the uncoded digital FM receiver; including, for example, baseband pulse shaping, click estimation and cancellation, envelope compensation and sequence estimation. For many years, numerous practitioners have sought useful methods for detecting and eliminating the FM-click noise to improve the FM receiver noise threshold. Despite these many efforts to attain this well-known and long-sought objective in the FM receiver art, the problem of optimum click detection is still open.
Even more than uncoded digital FM reception, encoded digital FM reception also suffers from the limiting effects of FM-clicks. For instance, a seminal paper by Simon (Marvin K. Simon, xe2x80x9cThe impact of mismatch on the performance of coded narrow-band FM with limiter-discriminator detection,xe2x80x9d IEEE Trans. Commun., vol. COM-21, pp. 28-36, January 1983) explores the theoretical performance of convolutionally-encoded narrow-band FM with LD detection and Viterbi decoding, using the theoretical methods introduced by Pawula. Simon finds that FM-clicks are the direct cause of the failure of decoding techniques based on soft decisions that assume Gaussian statistics at the LD output. Specifically, Simon used a Chernoff bounding technique to decouple the coding and the modulation and obtained some surprising theoretical results. For instance, he found that the FM-clicks create a xe2x80x9cmismatchxe2x80x9d between the coding channel and the decoding metric peculiar to the digital FM modulation. So the Viterbi decoding scheme does not provide xe2x80x9cmaximal likelihoodxe2x80x9d (ML) decoding for this modulation format in the presence of clicks. Accordingly, the expected performance improvement of soft decision decoding over hard decision decoding is inverted so that the hard decision decoding is better. In fact, the soft decision decoding performance is so degraded by the click effects that it cannot match even the uncoded system performance. Using an analog-to-digital converter (ADC) saturation level, the best theoretical soft decision decoding performance found was only 0.3 db better than the hard decision decoding performance. These results provide a clear example of the heavy performance penalty exacted by click noise in encoded digital FM receivers.
With this in mind, Simon proceeded to describe a theoretical FM receiver for which all FM-clicks are summarily removed by a xe2x80x9cgenie-aidedxe2x80x9d click detector, for which he suggests no useful embodiment. Simon shows that a xe2x80x9cgenie-aidedxe2x80x9d click detector can improve hard decision decoder performance by 1.3 dB and soft decision decoder performance by 3.3 db over a digital FM receiver for which no genie is available. Although Simon clearly shows the value of a xe2x80x9cpre-detectorxe2x80x9d for FM clicks, he neither teaches nor suggests any useful means for implementing such a mechanism for removing the FM-click noise component before decoding the digital FM signal at the receiver.
It is desirable to resolve this problem by providing a LD threshold detector that can eliminate the effects of FM-clicks on decoder BER performance. Until now, this has not been possible because of the well-known limitations discussed above. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention arises from the unexpectedly advantageous observation that, when the same limiter-discriminator (LD) output signal is presented to a sample-and-hold (SandH) detector and to an integrate and dump (IandD) detector, an error in one does not necessarily imply an error in the other because the SandH and IandD detector outputs are offset in time by one-half bit and they are not quite correlated. To the extent that the two detector outputs are uncorrelated, comparing the two detector output signals provides information sufficient to identify bit error locations, thereby allowing bit error correction in a subsequent decoder. With convolutional coding and Viterbi decoding, threshold-compensation and averaging of the two SandH and IandD detector output signals improves the receiver bit error rate (BER) performance by more than 3 dB over the soft-decision thresholded IandD detector alone, which, until now, was believed to be optimum in the art. Adding diversity to the coding and using envelope-compensation instead of threshold-compensation in the SandH detector improves BER performance even more.
It is a purpose of this invention to provide a new digital frequency-modulation (FM) LD receiver that can improve BER performance over that of the xe2x80x9coptimalxe2x80x9d FM LD receiver having an IandD detector.
It is an advantage of this invention that a receiver using a combination of the xe2x80x9coptimalxe2x80x9d IandD detector with the less-effective SandH detector improves BER performance by more than 3 dB over that of a receiver using an IandD detector alone.
In one aspect, the invention is a machine-implemented method for detecting the bit value corresponding to a data bit-interval of a digital FM receiver LD output signal including the steps of integrating the LD output signal over the data bit-interval to produce a first signal representing a first estimate of the corresponding bit value, producing a first bit detection signal representing a first threshold function of the first signal, sampling the LD output signal in the data bit-interval to produce a second signal representing a second estimate of the corresponding bit value, producing a second bit detection signal representing a second threshold function of the second signal, and combining the first and second bit detection signals to produce a final bit detection signal representing a final estimate of the corresponding bit value.
In another aspect, the invention is a digital FM receiver circuit having input means for receiving a digital FM signal representing a plurality of data bit-intervals each having a bit value, LD means coupled to the input means for demodulating the digital FM signal to produce a LD output signal having a value over a data bit-interval, IandD detector means coupled to the LD means for integrating the LD output signal over the data bit-interval to produce a first signal representing a first estimate of the corresponding bit value, first threshold means coupled to the IandD detector means for producing a first bit detection signal representing a first threshold function of the first signal, SandH detector means coupled to the LD means for sampling the LD output signal in the data bit-interval to produce a second signal representing a second estimate of the corresponding bit value, second threshold means coupled to the SandH detector means for producing a second bit detection signal representing a second threshold function of the second signal, and combining means coupled to the first and second threshold means for combining the first and second bit detection signals to produce a final bit detection signal representing a final estimate of the corresponding bit value.
It is another purpose of this invention to provide a new digital FM LD receiver that can identify and eliminate FM-clicks as they occur so that the associated bit errors can be corrected during the decoding process.
In yet another aspect, the method of the invention includes the steps of integrating the LD output signal over the data bit-interval to produce a first signal representing a first estimate of the corresponding bit value, producing a first bit detection signal representing to a first threshold function of the first signal, squaring the LD input signal envelope to produce a squared envelope signal, multiplying the squared envelope signal by the LD output signal to produce a product signal, sampling the product signal in the data bit-interval to produce a second bit detection signal representing a second estimate of the corresponding bit value, and combining the first and second bit detection signals to produce a final bit detection signal representing a final estimate of the corresponding bit value.
Another aspect of the invention is a digital FM receiver circuit having input means for receiving a digital FM signal representing a plurality of data bit-intervals each having a bit value, LD means coupled to the input means for demodulating the digital FM signal to produce a LD output signal having a value over a data bit-interval, IandD detector means coupled to the LD means for integrating the LD output signal over the data bit-interval to produce a first signal representing a first estimate of the corresponding bit value, first threshold means coupled to the IandD detector means for producing a first bit detection signal representing a first threshold function of the first signal, envelope squaring means coupled to the input means for squaring the digital FM signal envelope to produce a squared envelope signal, multiplier means coupled to the envelope squaring means and to the LD means for multiplying the squared envelope signal by the LD output signal to produce a product signal, SandH detector means coupled to the multiplier means for sampling the product signal in the data bit-interval to produce a second bit detection signal representing a second estimate of the corresponding bit value, and combining means coupled to the first and second threshold means for combining the first and second bit detection signals to produce a final bit detection signal representing a final estimate of the corresponding bit value.
It is an advantage of the receiver of this invention that, in the presence of FM-clicks, it provides receiver BER performance within 0.5 dB of the optimal theoretical performance for encoded signals without FM-click noise.
The foregoing, together with other features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.